Wear behaviour of multilayer coated WE43 magnesium alloy

Abstract

Perspective on decreasing the weight of the components in order to reduce CO2 emission has led the manufacturers in automotive and aerospace industries to consider the application of magnesium alloys owing to their high specific strength (strength to weight ratio). However, the limited strength of the magnesium alloys, especially above 120 °C, restricts their extensive usage. For this reason, instead of widely-used aluminium and zinc containing alloys such as AZ31 and AZ91, WE series magnesium alloys containing yttrium and rare earth elements have been developed for high temperature application up to 300 °C. Considering its high temperature and creep resistances, WE series alloys are attractive for aerospace and automotive industries. However, similar to the most widely used ones, this new alloy also suffers from poor wear and corrosion resistance due to low hardness and high surface reactivity. Therefore, in order to enhance wear and corrosion resistance of WE series alloys, modification of the surfaces by various techniques appears as an essential technical solution. In this respect, micro-arc oxidation (MAO) process, which forms magnesium oxide-based coatings, has been successfully employed on many commercially available grades of magnesium alloys. In the open literature, a few numbers of MAO studies have been conducted on the WE series alloys with the aim of improving the corrosion resistance. To the author's best knowledge, only one study is available in the open literature stating improvement in room temperature wear resistance of WE43 alloy by MAO process, which formed a coating consisting of magnesium oxide and magnesium silicate. However, magnesium oxide-based coatings did not provide sufficient protection against corrosion and wear due to their lower hardness and poor compactness. From this point of view, higher hardness of aluminium oxide (alumina) as compared to magnesium oxide motivated some researchers to fabricate multilayer coatings on magnesium alloys by applying cold spray and MAO processes sequentially. Thus, after the coating of magnesium alloys with aluminium or aluminium matrix composites by cold spray technique, MAO process has been employed to form aluminium oxide-based coatings at the outermost surface. The multilayer coating composed of alumina and aluminium or aluminium matrix composite layer exhibited superior wear and corrosion resistance than that of monolayer magnesium oxide-based coating. In the scope of this study, which was initiated mainly with the aim of improving high temperature wear resistance, multilayer (aluminium matrix composite/alumina) coating was fabricated on the WE43 magnesium alloy by subsequently applying CS and MAO process. While commercially available alumina particle reinforced aluminium matrix powder was used in the cold spray process, MAO processes were conducted in an aluminate-based electrolyte. In order to compare the properties of multilayer coat-ing with monolayer magnesium oxide coating, WE43 alloy was also subjected to the MAO process. Structural characterization of the fabricated coatings was done by using an optical microscope, energy dispersive spectroscopy equipped scanning electron microscope, and a X-ray diffractometer.